1. Field
The present invention relates generally to the reduction of dark current effects on imaging devices, and may be applied, for example, to portal imaging in conjunction with radiation therapy.
2. Description
Many types of conventional imaging devices create and represent image data using stored electrical charge. For example, known charge-coupled devices and amorphous silicon devices convert light to electrical charge and store the electrical charge for subsequent readout. In the case of amorphous silicon devices, a scintillator layer receives x-rays and generates light in proportion to the intensity of the received x-rays. An array of amorphous silicon photodiodes then converts and stores the generated light as electrical charge.
X-ray radiation may also be converted to and stored as electrical charge using amorphous selenium imaging devices. In operation, x-rays are absorbed by an array amorphous selenium photoconductors, which convert the x-rays directly to stored electrical charge. Due to the manner in which they operate, amorphous selenium imaging devices are said to produce image data through direct detection while amorphous silicon imaging devices are said to use indirect detection.
Because they rely on electrical charge to represent image data, the above-described imaging devices, and those that share similar characteristics, are vulnerable to phenomena that could adversely influence the conversion of light or other radiation to electrical charge and/or the storage of the electrical charge. Dark current is one such phenomenon.
For example, a photodiode of an amorphous silicon imaging device accumulates charge in proportion to an intensity of light received from an associated scintillator. After a specified time period, the accumulated charge is read in order to calculate the intensity of an image pixel associated with the photodiode. Accordingly, the accumulated charge is preferably directly proportional to the received light. The photodiode, however, requires a small bias voltage for proper operation. This bias voltage generates a small “dark current” that may cause a charge to accumulate within the photodiode that is unrelated to the intensity of the received light. This dark current thereby causes errors in the calculated intensity of the associated image pixel. Other imaging devices that convert radiation to electrical charge suffer from similar dark current problems.
Several approaches have been taken in an attempt to address the foregoing. According to one approach, imaging devices are designed so as to minimize dark currents and/or the effects thereof. This approach involves the development of new semiconductor devices, doping techniques and circuit designs, and therefore can be quite costly. Moreover, no known technique has been shown to efficiently and satisfactorily address dark current issues.
Another approach applies image processing techniques to each image frame that is produced from electrical charges read from an array of imaging elements. Known as offset correction, this approach involves acquiring image frames during a period of non-irradiation, calculating an average image frame from the acquired frames, and subtracting the average image frame from each frame acquired during subsequent radiation of the imaging elements. The averaged image frames are preferably acquired at a same rate as the subsequently-acquired frames so as to better approximate the effect of dark current on the subsequently-acquired frames. Since the extent of dark current effects vary across imaging devices, imaging devices are often sold with customized software for performing offset correction.
Offset correction often fails to provide suitable reduction of dark current effects, alone or in combination with the first approach described above. Therefore, additional or alternative systems for reducing dark current effects are desirable.